When people ask me about my age, I say, "Guess."
I'm 21, but look a lot younger. I've had people (recently) think I'm 12, 15, 16, etc. So I make them guess, just so I can have more fodder to make fun of myself. And them, of course.
Someone told my mom last week that she thought my mom was in her mid thirties -- she's 48 with two kids in college and one already out.

And people have always told my dad, "You look too young to be married/have kids/have kids in school/have kids in high school/have kids in college/have a son getting married." So I guess I'm doomed to look young for the rest of my life...I know, I know, I'll appreciate it when I'm older.
Bethy (who doesn't really mind it except when it's a cute guy who thinks she's more kid sister material than potential smoochie material. <g>)